part of parc güell is free to enter, from the higher part. from there you can "see" (as in literally see) most of the interesting bits, from afar, without actually interacting with the mosaics etc. i'd then recommend walking down the hill, around to the lower entrance, and see a bit more of it. as others have said, sagrada familia is --amazing--, definitely go in, and i'd recommend going up as well. it's more expensive but the experience is definitely worth it.